Alternator, Battery

Have the battery and the alternator tested.

I know how to check the battery, but do you know how can I check the alternator?

You can start the car and then disconnect the battery. If the car keeps running the alternator is most likely still good, if it dies the alternator may be the issue. You can also remove the alternator and have it tested (make sure to disconnect the battery before working on the alternator).

DO NOT disconnect the alternator while the engine is running. This "Redneck" test can send a voltage spike into the electrical system that can damage the ECU. You can drive the vehicle to an auto parts store and have the alternator tested on the vehicle or you can remove it and take it to have it tested. The B+ supply (large connection) should have battery voltage supplied to it at all times, even when the key is in the off position. If battery voltage at the alternator is missing, then there is a problem somewhere along the wire that supplies the alternator with power.

Battery is 12.5 volts. When the car starts, the Alternator puts out 14.3 volts to charge the battery. Thats ok